Would it be allowed under UK law to work in a full-time job but still do a PhD in Maths and Finance?

I'm hoping to become a full-time trainee accountant once I finish my Master's Degree in mathematics and gain a Level 4 Diploma as an accounts technician, but sitting a PhD would be additional income of �13K-�19K per year! Doing both would make me quite well-off for a young man, but would it be allowed?